1. Field of the Invention
The present invention relates to a water electrolysis system.
2. Discussion of the Background
Hydrogen gas can be used as fuel gas to cause a polymer electrolyte fuel cell to generate electric power. Typically, a water electrolysis device is employed in producing hydrogen gas. The water electrolysis device uses a solid polymer electrolyte membrane (ion exchange membrane) to decompose water to generate hydrogen (and oxygen). Electrode catalytic layers are provided at opposite surfaces of the solid polymer electrolyte membrane to form an electrolyte membrane electrode assembly, and an anode-side feed and a cathode-side feed are disposed at both sides of the electrolyte membrane electrode assembly. This configuration forms a unit.
The water electrolysis device includes a plurality of such units being stacked. In this state, a voltage is applied to both ends in the direction in which they are stacked, and water is supplied to the anode-side feed. At the anode of each of the electrolyte membrane electrode assemblies, water is decomposed and hydrogen ions (protons) are formed, the hydrogen ions permeate through the solid polymer electrolyte membrane and move to the cathode, they combine with electrons, and hydrogen is formed. At the anode, oxygen generated together with the hydrogen is discharged with redundant water from the unit.
The above-described water electrolysis device produces hydrogen containing moisture, and thus it is necessary to remove the moisture from the hydrogen to obtain hydrogen in a dry state, for example, at or below 5 ppm (hereinafter referred to also as dry hydrogen). One known example of a technique that meets such a need is a gas-liquid separation device of a water electrolysis device disclosed in Japanese Unexamined Patent Application Publication No. 8-144078.
This gas-liquid separation device includes a first gas-liquid separation tank 1a and a second gas-liquid separation tank 1b, as illustrated in FIG. 7. The upper portion of the first gas-liquid separation tank 1a communicates with an anode chamber of a water electrolysis device (electrolytic cell) 3 through a gas and liquid inlet tube 2a. Oxygen generated at the anode of the water electrolysis device 3 and pure water supplied to the anode chamber of the water electrolysis device 3 are transported as mixture to the first gas-liquid separation tank 1a through the gas and liquid inlet tube 2a. 
The upper portion of the first gas-liquid separation tank 1a and the upper portion of the second gas-liquid separation tank 1b are connected with a gas communicating tube 2b for oxygen disposed therebetween, and oxygen gas obtained by gas-liquid separation in the first gas-liquid separation tank 1a flows into the second gas-liquid separation tank 1b. The lower portion of the first gas-liquid separation tank 1a and the lower portion of the second gas-liquid separation tank 1b are connected with a liquid communicating tube 2c for pure water disposed therebetween, and pure water obtained by gas-liquid separation in the first gas-liquid separation tank 1a flows into the second gas-liquid separation tank 1b. 
The second gas-liquid separation tank 1b is provided with a bypass path 1d for use in controlling a liquid level of a liquid component obtained by separation in the first gas-liquid separation tank 1a. The bypass path 1d has a substantially U shape and extends substantially vertically along the wall surface of the tank. Optical sensors 4a and 4b for use in controlling a liquid level are provided on the outside of the bypass path 1d at locations corresponding to preset upper and lower limit positions, respectively, for the liquid level in the second gas-liquid separation tank 1b. 
A high-pressure water electrolysis system for generating hydrogen of high pressure (e.g., approximately 35 MPa) at the cathode is used in a water electrolysis system. In this case, high-pressure hydrogen containing moisture is introduced into a gas-liquid separation device connected to the cathode of the water electrolysis device.